The main purpose in providing this type is to verify that standard library
functions are found via a using declaration - bringing those functions into
the current scope - and not just because they happen to be in global scope.

In order to ensure that a call to say pow
can be found either via argument dependent lookup, or failing that then in
the std namespace: all calls to standard library functions are unqualified,
with the std:: versions found via a using
declaration to make them visible in the current scope. Unfortunately it's
all to easy to forget the using
declaration, and call the double version of the function that happens to
be in the global scope by mistake.

For example if the code calls ::pow rather than std::pow, the code will cleanly
compile, but truncation of long doubles to double will cause a significant
loss of precision. In contrast a template instantiated with std_real_concept
will only compile if the all the standard
library functions used have been brought into the current scope with a using
declaration.

real_concept is an archetype
for user defined real types,
it declares its standard library functions in its own namespace: these will
only be found if they are called unqualified allowing argument dependent
lookup to locate them. In addition this type is useable at runtime: this
allows code that would not otherwise be exercised by the built-in floating
point types to be tested. There is no std::numeric_limits<> support
for this type, since numeric_limits is not a conceptual requirement for
RealTypes.

The class template DistributionConcept
verifies the existence (but not proper function) of the non-member accessors
required by the Distribution
concept. These are checked by calls like

v = pdf(dist, x); // (Result v is ignored).

And in addition, those that accept two arguments do the right thing when
the arguments are of different types (the result type is always the same
as the distribution's value_type). (This is implemented by some additional
forwarding-functions in derived_accessors.hpp, so that there is no need for
any code changes. Likewise boilerplate versions of the hazard/chf/coefficient_of_variation
functions are implemented in there too.)